This invention relates to a distortion compensating apparatus and, more particularly, to (1) a distortion compensating apparatus capable of limiting amplitude when it appears that control which exceeds output limits will be carried out and capable also of exercising a phase tracking operation even when amplitude has been limited, and (2) a distortion compensating apparatus so adapted that the amplitude of a signal fed back from a transmission power amplifier is controlled so that a limit value will not be exceeded, and so adapted that distortion can be compensated for in stable fashion.
Frequency resources have become tight in recent years and in wireless communications there is growing use of high-efficiency transmission using digital techniques. In instances where multilevel amplitude modulation is applied to wireless communications, a vital technique is one which can suppress non-linear distortion by linearizing the amplitude characteristic of the power amplifier on the transmitting side and reduce the leakage of power between adjacent channels. Also essential is a technique which compensates for the occurrence of distortion that arises when a attempt is made to improve power efficiency by using an amplifier that exhibits poor linearity.
FIG. 45 is a block diagram illustrating an example of a transmitting apparatus in a radio according to the prior art. Here a transmit-signal generator 1 transmits a serial digital data sequence and a serial/parallel (S/P) converter 2 divides the digital data sequence alternately one bit at a time to convert the data to two sequences, namely an in-phase component signal (also referred to as an “I signal”) and a quadrature component signal (also referred to as a “Q signal”). A DA converter 3 converts the I and Q signals to respective analog baseband signals and inputs these to a quadrature modulator 4. The latter multiplies the input I and Q signals (the transmit baseband signals) by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90° and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. A frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier 6 power-amplifies the carrier output from the frequency converter 5. The amplified signal is released into the atmosphere from an antenna 7.
In mobile communications based upon W-CDMA and PDC (Personal Digital Cellular) techniques, etc., the transmission power of the transmitting apparatus is a high 10 mW to several watts, and the input/output characteristic [distortion function f(p)] of the transmission power amplifier is non-linear, as indicated by the dotted line in FIG. 46A. Non-linear distortion arises as a result of non-linear characteristics, and the frequency spectrum in the vicinity of a transmission frequency f0 develops side lobes, as shown in FIG. 46B, leakage into the adjacent channel occurs and this causes interference between adjacent channels. More specifically, owing to non-linear distortion, there is an excessive increase in power that causes transmitted waves to leak into the adjacent frequency channel, as shown in FIG. 46B. The leakage power is described on the basis of ACPR (Adjacent Channel Power Ratio). ACPR is the ratio between the power of the channel of interest, which is the area of the spectrum between the one-dot chain lines A and A′ in FIG. 46B, and the adjacent leakage power, which is the area of the spectrum between the two-dot chain lines B and B′, that flows into the adjacent channel. Such leakage power constitutes noise in other channels and degrades the quality of communication of these channels. Such leakage must be limited to the utmost degree.
Leakage power is small in the linear region (see FIG. 46A) of the power amplifier and large in the non-linear region. Accordingly, it is necessary to broaden the linear region in order to obtain a transmission power amplifier having a high output. However, this necessitates an amplifier having a performance higher than that actually needed and therefore is inconvenient in terms of cost and apparatus size.
With an ordinary amplifier, power added efficiency in the linear region is low, as indicated by FIG. 47. Power load efficiency, which is the percentage (%) of the difference (Pout−Pin) between output power Pout and input power Pin with respect to the rated power of the amplifier, is the portion given off as heat. To obtain the necessary transmission power, therefore, it is required that a large amount of power be consumed. This is inconvenient in terms of power efficiency. Thus it is essential to use the amplifier in the non-linear region in order to hold down the amount of power consumed. As mentioned above, however, distortion increases and degrades the ACPR. A device that compensates for distortion of transmission power and enables use of an amplifier in a region of excellent power added efficiency is a wireless apparatus (linearizer) having a distortion compensating function. The Cartesian loop method and polar loop method, etc., have been proposed as techniques for effecting distortion compensation by feedback and these methods succeed in suppressing the distortion of power amplifiers.
FIG. 48 is a block diagram of a transmitting apparatus having a digital non-linear distortion compensating function that employs a DSP. Here digital data (a transmit signal) sent from the transmit-signal generator 1 is converted to I and Q signals by the S/P converter 2. These signals enter a distortion compensator 8 constituted by a DSP. As illustrated in FIG. 49, the distortion compensator 8 functionally comprises a distortion compensation coefficient memory 8a for storing distortion compensation coefficients h(pi) (i=0˜1023) conforming to power levels 0˜1023 of a transmit signal x(t); a predistortion unit 8b for subjecting the transmit signal to distortion compensation processing (predistortion) using a distortion compensation coefficient h(pi) that is in conformity with the level of the transmit signal; and a distortion compensation coefficient calculation unit 8c for comparing the transmit signal x(t) with a demodulated signal (feedback signal) y(t), which has been obtained by demodulation in a quadrature detector described later, and for calculating and updating the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will approach zero.
The distortion compensator 8 subjects the transmit signal x(t) to predistortion processing using the distortion compensation coefficient h(pi) that conforms to the power level of the transmit signal x(t), and inputs the processed signal to the DA converter 3. The latter converts the input I and Q signals to analog baseband signals and applies the baseband signals to the quadrature modulator 4. The latter multiplies the input I and Q signals by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90° and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. The frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier 6 power-amplifies the carrier output from the frequency converter 5. The amplified signal is released into the atmosphere from an antenna 7.
Part of the transmit signal is input to a frequency converter 10 via a directional coupler 9 so as to undergo a frequency conversion and then be input to a quadrature detector 11. The latter multiplies the input signal by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90°, reproduces the I, Q signals of the baseband on the transmitting side and applies these signals to an AD converter 12. The latter converts the applied I and Q signals to digital data and inputs the digital data to a distortion compensator 8. By way of adaptive signal processing using the LMS (Least Mean Square) algorithm, the distortion compensator 8 compares the transmit signal before the distortion compensation thereof with the feedback signal modulated by the quadrature detector 11 and proceeds to calculate and update the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will become zero. The transmit signal to be transmitted next is then subjected to predistortion processing using the updated distortion compensation coefficient and the processed signal is output. By repeating this operation, non-linear distortion of the transmission power amplifier 6 is suppressed to reduce the leakage of power between adjacent channels.
FIG. 50 is a diagram useful in describing distortion compensation processing by an adaptive LMS. A multiplier (which corresponds to the predistortion unit 8b in FIG. 49) 15a multiplies the transmit signal (the quadrature-modulated signal) x(t) by a distortion compensation coefficient hn−1(p) and applies its output to a transmission power amplifier 15b having a distortion function f(p). A feedback loop 15c feeds back the output signal y(t) from the transmission power amplifier 15b and an arithmetic unit (amplitude-to-power converter) 15d calculates the power p [=x(t)2] of the transmit signal x(t). A distortion compensation coefficient memory (which corresponds to the distortion compensation coefficient memory 8a of FIG. 49) 15e stores the distortion compensation coefficients that conform to the power levels of the transmit signal x(t). The memory 15e outputs the distortion compensation coefficient hn−1(p) conforming to the power p of the transmit signal x(t) and updates the distortion compensation coefficient hn(p) by a distortion compensation coefficient hn(p) found by the LMS algorithm.
A complex-conjugate signal output unit 15f has the output of the feedback system 15c applied thereto. A subtractor 15g outputs the difference e(t) between the transmit signal x(t) and the feedback (demodulated) signal y(t), a multiplier 15h performs multiplication between e(t) and u*(t), a multiplier 15i performs multiplication between hn−1(p) and y*(t), a multiplier 15j multiplies the output of the multiplier 15h by a step-size parameter μ, and an adder 15k adds hn−1(p) and μe(t)u*(t). Delay units 15m, 15n, 15p add a delay time to the input signal. The delay time is equivalent to the length of time from the moment the transmit signal x(t) enters to the moment the feedback (demodulated) signal y(t) is input to the subtractor 15g. The complex-conjugate signal output unit 15f and the multipliers 15h, 15i and 15j construct a rotation calculation unit 16. A signal that has sustained distortion is indicated at u(t). The arithmetic operations performed by the arrangement set forth above are as follows:hn(p)=hn−1(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=hn−1(p)x(t)f(p)u(t)=x(t)f(p)=h*n−1(p)y(t)P=|x(t)|2where x, y, f, h, u, e represent complex numbers and * signifies a complex conjugate. By executing the processing set forth above, the distortion compensation coefficient h(p) is updated so as to minimize the difference e(t) between the transmit signal x(t) and the feedback (demodulated) signal y(t), and the coefficient eventually converges to the optimum distortion compensation coefficient h(p) so that compensation is made for the distortion in the transmission power amplifier.
FIG. 51 is a diagram showing the overall construction of a transmitting apparatus expressed by x(t)=I(t)+jQ(t). Components in FIG. 51 identical with those shown in FIGS. 48 and 50 are designated by like reference characters.
As mentioned above, the principle of digital non-linear distortion compensation is to feed back and detect a carrier obtained by quadrature modulation of a transmit signal, digitally convert and compare the amplitudes of the transmit signal and feedback signal, and update the distortion compensation coefficient based upon the comparison. In accordance with this method of digital non-linear distortion compensation, distortion can be reduced. As a result, the ACPR requirement is satisfied (i.e., leakage power can be held low) with a high output and even with operation in the non-linear region, and the power added efficiency can be improved, thus making it possible to reduce power consumption. Further, the amount of heat evolved can be reduced by the improvement in power added efficiency, thereby mitigating the need for measures to deal with such heating. The end result is an apparatus of smaller size.
When distortion occurs, the signal undergoes amplitude distortion and phase distortion simultaneously. The reason for this is that when a transmit signal that has been compensated for distortion exceeds the compensation amplitude limits of the distortion compensating circuit, the signal has its amplitude limited to the threshold value of the distortion compensating apparatus, the amplitude value becomes stuck at the upper-limit value of the distortion compensating apparatus and phase control becomes impossible.
Though a transmission power amplifier has a non-linear characteristic owing to saturation, the amplifier is used in a state as close to saturation as possible in view of transmission efficiency, as mentioned above. On the other hand, since the distortion compensating apparatus controls distortion compensation in such a manner that the characteristic is made linear, the distortion compensation coefficient hn(p) becomes gradually larger when the apparatus is used in a state near saturation. As a consequence, there is a rise in the level of the transmit signal x(t)*h(p) (where * signifies complex multiplication) after the distortion compensation thereof, the dynamic range of the DA converter is exceeded and the amplitude of the output of the DA converter becomes distorted. As a result, the transmit signal comes to contain harmonic components, phase becomes distorted as well as amplitude and leakage between adjacent channels occurs. This means that the spectrum characteristic will be out of specs.
FIG. 52 is a diagram useful in describing problems encountered with the conventional phase compensating apparatus. The dashed line LM in FIG. 52 indicates the dynamic range of the DA converter 3 (i.e., the DA converter limit). Distortion will not occur if the level of the transmit signal x(t)*hn(p) output from the predistortion unit of the distortion compensating apparatus is within the DA converter limit LM. However, if the distortion compensation coefficient h+1(p) for the transmit signal x(t) increases owing to distortion compensation processing, then x(t)*hn+1(p) will exceed the DA converter limit LM, the amplitude will be clamped to the DA converter limit LM, harmonic components will be produced and phase will become distorted, as mentioned above.
More specifically, in the region where the power amplifier has a high degree of non-linearity, the amplitude difference e(t) between the transmit signal x(t) prior to correction and the feedback signal increases without an increase in the amplitude of the feedback signal y(t) regardless of the fact that it is being attempted to enlarge the amplitude by distortion compensation. If the amplitude difference takes on a large value, the distortion compensator 8 judges that distortion compensation is not being carried out as desired and enlarges the distortion compensation coefficient hn+1(p) in such a manner that the difference signal e(t) becomes smaller. As a result, the signal amplitude after distortion compensation thereof is caused to increase and, consequently, the signal amplitude exceeds the limit value (the limit DM of the DA converter limit 3). This means that the amplitude of a signal whose amplitude has exceeded the limit value takes on a constant amplitude value, resulting in loss of significant components (amplitude and phase) of the signal. As a consequence, not only compensation of the amplitude component but also compensation of the phase component can no longer be carried out. In other words, an obstacle which arises is that distortion compensation does not operate normally.
As a result of the foregoing, when amplitude exceeds the DA converter limit LM, it becomes impossible to control both amplitude and phase and the distortion characteristic becomes worse than that when no distortion compensation is applied.